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How can pilots lacking instruments interpret vectors given by ATC?
This video is about European Air Transport flight 2939, a Boeing 737, which "lost all the instruments of the aircraft" shortly after takeoff.

BCS2939: We are flying heading now, and we don't have even altitude and anything, so if you can, give vectors back to the field, please.
  Approach: Roger, Eurotrans 2939, turn right heading 360 and you can climb to 3000 feet.
  BCS2939: Right turn 360, 3000 feet. -pause- We are almost in visual, we don't even have altimeters.

Despite the lack of instruments the pilots manage to fly the headings given by Shannon Approach and land safely.
It seems that the instrument panels of the 737 are entirely digital:

I understand what vectors are:

A vector is defined by a direction and magnitude. In aviation these represent your heading (the direction) and your speed (the magnitude). However, in normal aviation usage "vector" only refers to the heading and other nomenclature is used to assign/report speeds.

The 737's cockpit seems to lack an analog compass which would survive a power outage like this one:

How might the pilots have flown the headings given by ATC (as the visualisation shows they did) without a compass or altimeter?

You can't see it in the picture you have (only the bottom black part is visible), but it does have a liquid filled compass at the top of the center vertical column separating the two windows.

Sounds like they lost their pitot-static system, which would mean no altitude or airspeed, but they'd still have normal heading & attitude.  Losing all the displays would be pretty bad, although if it was only the displays that were lost, they'd still have altitude, airspeed, and heading (and attitude) on the standby instruments.  Since they were able to maintain control of the aircraft in IMC, they had some sort of attitude indication, so this sounds to me like a case of losing the pitot-static system. Which, while plenty bad, should be survivable. A very bad day at the office, though.

Even if there was no magnetic compass in the cockpit, they could use a smartphone which is able to provide both bearings and GPS speed and altitude, or even just a Tomtom. I'm sure everybody, including EASA and FAA, would agree a smartphone in receive mode is less dangerous than losing air data.

@mins:  Try using a handheld compass in a vehicle some time.  It is wildly variable depending upon the exact millimetre of positioning relative to ferrous objects.  When an aircraft compass is installed, it has to be bolted into final position and then "adjusted" which compensates for all the objects in the cockpit, engine, fuselage, etc. which modify the magnetic field sensed by the compass.  A smartphone using GPS information only and ignoring its magnetic compass would work well though.

@wallyk: That's true, but a handled GPS usually knows the North direction without using a magnetic sensor. It's also true that when one doesn't have visual references to perform a 30° right turn, an uncompensated compass is better than nothing. The B737 IRS can also be used in Track/GS or Heading mode to approximate the North.

I have a 737-max magnetic compass on my desk in front of me right now and a magnetic compass is required by law. So you may have some bad information. Flight in the clouds cannot be done without at least one instrument to give an indication of straight vs turning. Vectors can say "turn right, stop turn, climb, stop climb,..." but some instrument is needed to avoid total disorientation and spiral dives.

14 CFR Part 25 describes the requirements for an airliner like the 737 to be certified by the FAA.

§25.1303   Flight and navigation instruments.
(a) The following flight and navigation instruments must be installed so that the instrument is visible from each pilot station:
...
(3) A direction indicator (nonstabilized magnetic compass).

Indeed the 737 has a magnetic compass, centrally located above the windshield. It's just hard to see in the picture you have, as it's hidden behind the overhead console controls. This is a very typical location for the compass in many large aircraft (but not all), as it's centrally located, allows both pilots to see it, and will have less interference from other instruments.

Even the newest version, the 737 MAX, has a compass.

Images Source

As fooot has described, the aircraft has a mechanical compass, which will work even if the electrical systems fail.
Though unlikely, it is possible to imagine a situation where the crew of an aircraft loses all heading indicators. In such a situation, ATC can still provide radar vectors. ATC will instruct the crew which turn rate to use, and when to start and stop their turn, based on observations on the ATC radar. The radio phraseology will sound like this:

Make all turns rate one (or [number] degrees per second), start and stop all turns on the command "now"
Turn left (or right) now
Stop turn now

It's nice to see that everything's been thought of :)

@hobbs ATC can even figure out how to get somebody who "has only flown once before as a passenger" to make a successful landing, if they have to: http://www.bbc.co.uk/news/uk-england-humber-24450534. ("Successful" in the sense "that he could walk away from it" - touching down nose-wheel-first and bouncing the prop on the runway are just details!)


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What is the relation between geodesic airframe and geodesic curves?
That is certainly a beginner question but I cannot find the answer anywhere.
The Wikipedia page on geodesics says that they are the shortest path between two points on a geometric object (surface, manifold,...). It also  mentions geodesic airframes as a real-life application. However, the linked page about geodesic airframes does not seem to deal with geodesic curves. It is mostly about about a structure made by triangles or other polygons. A related question explaining why geodesic frames are no longer in use is answered only with this point of view.
Hence, my question: are geodesic frames related to geodesic curves? If yes, in what way?
My intuition is that geodesic curves are used to design the shape of the "perfect" frame (with respect to some constraints), and that the polygonal structure is an approximation of this perfect shape.

See also Barnes Wallis Foundation and this explanation: "Two geodesic arcs intersect on a curving surface [the fuselage] in such a way that the load on each is cancelled out."

The GIF from the Geodesic dome wiki is also built up from polygons, so i'm not use if that's necessarily an approximation

Before answering the questions, let's try to see how simple is the notion of a geodesic. In planar construction, we use latices to rigidify a structure, e.g. a truss:

The structure is made of straight segments that don't deform when axial forces are applied. If we switch from the plane to an arbitrary curved surface, something similar can be done, but geodesics take the place of the straight lines (actually the line is the geodesic for the plane).
The idea behind a geodesic construction is that its rigidity is obtained from segments on the surface of the truss, and the inner volume is left empty and available for other uses.

What is the relation between geodesic airframe and geodesic curves?
Are geodesic frames related to geodesic curves? If yes, in what way?

Geodesic airframes are indeed constructions made of geodesics, a technique first applied to aviation by Barnes Wallis. From the Barnes Wallis Foundation, and article Geodetic Aircraft Design:

A geodetic (or geodesic) airframe makes use of a space frame formed
  from a spirally crossing basket-weave of load-bearing members. By
  having the geodesic curves form two helices at right angles to one
  another, the members became mutually supporting in a manner that the
  torsional load on each cancels out that on the other. In addition to
  being comparatively light and strong, the fact that the geodetic
  structure was all in the outer part of the airframe meant that the
  centre was a large empty space, ready to take payload or fuel.

A close-up view of the geodetic structure designed by Barnes Wallis (source)

Vickers Wellington's under construction (source)

[The wikipedia article] does not seem to deal with  geodesic curves. It is mostly about about a structure made by triangles or other polygons

Geodesics are curved lines. Using 2 or 3 geodesics on a closed surface creates repetitive and symmetrical curved polygons, e.g. curved triangles and curved lozenges. Providing the supporting geodesics are angled so that "the torsional load on each cancels out that on the other", the resulting geometry belongs to the family of geodesic modeling.
Whether the actual construction is an approximation of the perfect model is a different engineering/costing problem. This is related to the actual cancellation of the torsional loads after approximation and the effect of the residual loads on the structure. This residual load could potentially be absorbed by additional engineering device if appropriate. This dome is made of straight segments only, the triangles are planar.

The geodesic dome at Amundsen Scott South Pole Station by Ernie Mastroianni (source)

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

sort: votes


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Are there additional electrical grounding methods besides static wicks?
Static dischargers have high electrical resistance, and work via corona discharge.
So if I understand correctly, what wicks do is limit the voltage that the aircraft builds up. But as the wick is ionizing the air around, I guess this voltage is rather high.
Do commercial, fixed-wing aircraft need any additional grounding method once on the ground, when the passengers are boarding/unboarding for example?. How it's done?
Related: This question is about grounding for hangar storage, and also explains grounding for refueling operations

This question is currently broad, as the methods differ depending on the use case. For example, helicopters vs. airplanes, in flight vs. on the ground, etc. It would probably be helpful to narrow this down a bit.

@reirab but the question already explicitly limits the scope to comercial, fixed-wing aircraft [...] once on the ground

@Federico Well, that part specifically said "for example." The static wicks mentioned in the title don't really apply to that scenario. If the question is just about that part, then I agree that it would be sufficiently narrow, but the question should be reworded (and retitled) to indicate that. The existing answers don't seem to have read it as being limited to that scenario, either.

@Federico commercial, fixed wing aircraft could encompass the vast majority of aircraft produced. Commercial is an undefined, very broad adjective when applied to aircraft types. The question could be narrowed to airliners or even large passenger aircraft would be helpful.

"I guess this voltage is rather high": It is, but this is not this voltage (volts) that makes grounding desirable, it's the high number of unbalanced ions (coulombs) that can create at any moment a significant current (amperes). Our hairs create high voltages (25 kV), but the charge is insignificant, we don't need our head to be grounded :-) That's a relief.

Many aircraft tyres conduct electricity which also dissipates charges.  The compound contains "carbon black" or graphites.
In the special case of helicopters using a hook to pick up a load from a hover, the wise ground handler will use a rod on the end of a grounded cable to earth the charge.  Helicopters can build up a lot of static and many a ground handler has been left with smoking underpants by not heeding this advice.

When fueling a grounded connection is established. Or at the very least the fuel tank is electrically bonded to the aircraft before the fuel line is connected by a clamp on a post on the landing gear.

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What is a Vmc roll?
Many aviation discussions of the TransAsia GE235 accident include comments like "that was a Vmc roll". What is a Vmc roll and how should you avoid it and/or recover from it?

I'm sure someone else will come along and give you a better answer with illustrations and everything, but until then, here goes :)
Multi engine aircraft usually have their engines off to either side of the fuselage. This means that if one engine fails, the other engine will produce what's called asymmetric thrust (one engine is stronger than the other). This in turn, because it's off to the side of the aircraft will cause it to yaw in the direction of the dead engine. Should the dead engine also have its prop windmilling, it'll cause more drag than if the prop blades were turned parallel to the wind and thus aggravate the situation by not only not producing thrust but actually acting as a speed brake.
To counteract this and continue flying straight you'd have to apply the aircraft's rudder. The rudder is an airfoil sticking out into the wind in order to yaw the aircraft, thus if the amount of air flowing over it is reduced, the effectiveness of the rudder is reduced. This has the effect that there's a minimum airspeed (i.e. minimum amount of airflow over the rudder) that's enough to allow the rudder to counteract the asymmetric thrust as well as some other properties like engine torque. This speed is called $v_{mc}$, or minimum controllable airspeed (there are actually two of these, one airborne and one on the ground). If speed drops below this airspeed, the plane will start to increase the yaw away from the live engine, to the point where eventually the outer wing produces more lift than the inner wing and the aircraft rolls over.
That's the $v_{mc}$ roll. Avoiding it is a matter of keeping the airspeed up to maintain rudder authority, and/or reducing thrust to the live engine while regaining airspeed. Further steps, like identifying the failed engine and feathering its prop, as well as flying with a slight bank towards the live engine should be undertaken as well as part of the recovery.
Furthermore it's advisable to never turn into the dead engine, as the live engine will make it harder to stop the turn. Turning into the live engine will make it harder to start the turn, but the engine will help you get out of it.
Since the engines usually spin in the same direction, the effect of torque differs, the torque of one engine aggravates the situation, where the torque of the others alleviates it, thus $v_{mc}$ is specified as the minimum controllable speed if the latter (known as the critical engine) fails and the prop continues to windmill, i.e. the worst possible aerodynamic scenario in the case of a single engine failure barring an uncontained failure causing damage or otherwise changing the aerodynamic properties of the aircraft.

Kevin precisely answers the question, you describe it well for laymen. If we could just get the two answers combined into one...

@FreeMan this is SE, there's nothing wrong with more than one answer ;)

That's true @falstro, sadly, you won't both get credit for great answers.

@FreeMan True but the 15-point bonus for having the accepted answer is only a vote and a half's worth of reputation. I agree it would often be nice to accept two answers, though.  (More than two would probably be getting a little much.)

It's more correct to say this is an unintended roll induced by flying at a speed below Vmc.
Vmc (V=Velocity, mc = minimum controllable) is the minimum speed a multi-engine aircraft can remain in control in case of a single engine failure. When an engine quits, thrust is no longer symmetrical. Rudder and a little bit of aileron are used to keep the plane level. E.g. If the left engine quits, the plane has a tendency to yaw left and roll left.
Effectiveness of control surfaces increases with speed. When the speed drops too low, the control surfaces may be unable to balance the plane even at full deflection.
Pilots are trained to keep the speed above Vmc at all times. For example, during the takeoff roll, it is a bad idea to lift off the runway before the airspeed reaches Vmc. If the plane pitches up too much during a climb, its speed may also fall below Vmc.

Maybe to provide further info, when the engine quits the plane has direct tendency to yaw left, and compensates using the rudder at the vertical plane. This deflection creates the tendency to roll. Just to provide some extra mechanics :)

The airplane has an engine failure on the left side. The wing on the right side has high velocity air blown over the wing providing more lift and keeping the boundary layer from slowing down and separating. The pilot counteracts the the rolling moment from the high lift right wing by using right aileron. Now the aileron on the left side is deflected down, increasing the effective angle of attack of the left wing, a wing that has nothing to sweep away the slowing boundary layer. Now he is using right rudder to try to keep the plane going straight. His mind is thinking he has the plane under control, wings level and plane going straight. But the plane is side slipping. This is the equivalent of bottom rudder. The skidding causes the plane to slow down because it can hardly fly when it it isn't skidding. The slowing down causes the rudder to be less effective, the skidding increases, that means more aileron is needed due to the dihedral effect.  Finally the effective angle of attack of the left wing is high enough for the stall to begin and the wing starts down. The right ring is rock solid, it will never stall. The left wing dropping increases the angle of attack more and the rollover is rapid.

The Vmc roll, specifically, is a rapid roll (not yaw) below Vmc. It occurs primarily because a thrusting engine located on a wing decreases the stall speed significantly. Therefore, as airspeed decreases, a moment occurs when the wing with non-thrusting engine stalls, while the other wing continues to produce lift. At that point the airplane quickly rolls.
Note that 1) for this asymmetry to happen you need a rather small amounts of thrust, and 2) the stall speed of one wing grows. So, if you think that you made it once the trottle is cut and the rudder deflection is way down, and try to fly a usual landing, then you will die. See this:
 https://www.youtube.com/watch?v=YZIzEtHzbNU
This is why all the yaw-centered explanations of Vmc roll are hazardous and you should reject them as soon as possible. It's not yaw that will kill you, it's the roll -- and it can happen from a minimal yaw.

There's a lot of vmc talk in the comments of that YouTube video, but to be honest, that looks like a classic base to final bottom rudder stall/spin, not an engine failure. Do you have a link to the accident report?

@falstro - A small amount of detective work revealed this: http://aviation-safety.net/wikibase/wiki.php?id=145223.  No NTSB report (broken link)

One of the most important points is Vmca is for straight flight only and does not give protection for turns.  Also when the tail is designed the engineer is by FAS,s allowed up to 5 deg of bank to offset the side slip.  This allows the tail to be slightly smaller saving weight.  The wings should never be kept level but the design bank maintained.  Look in your pilot handbook.  Normally in the engine out there will be a slight reference to a bank angle with no explanation.  This is the bank angle the designer used and the flight test group used to certify the numbers.  Vmca has nothing to do with maneuvering. It is for straight flight only.  Any bank greater or less will raise the Vmca speed significantly.  On takeoff with wings level the Vmca speed in most light twins is 8 to 10 knots faster than published Vmca.

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Is there a practical use of streaming data received from meteorological sensors installed on aircraft?
I am not the practitioner in the field of aviation, and my knowledge is mainly theoretical. Therefore I would be happy to get a feedback from practitioners on the following topic: I was reading about the use of aircraft as meteorological sensors. I wonder how this streaming sensor data is currently used by pilots? Is it validated for logical consistency?
Moreover, I was thinking about possible applicability of such data, and in my opinion it could be used by Air Traffic Controllers for enabling/disabling airspace sectors for free routing (direct flight). Does it make sense from a practical point of view? This data could be shared via System Wide Information Management among different aviation stakeholders.

Related: See TAMDAR and more on TAMDAR. Also AMDAR, and more on AMDAR. Both based on ACARS. The goal is more to provide data to weather organizations that need a smaller measurement mesh than the current one with only weather stations and balloons. FAA and other ATC providers then can use the weather reports.

Yes, such a system would be beneficial to pilots, ATC, and meteorologists.
Pilots are very concerned with meteorological phenomena like icing and turbulence (for safety-of-flight reasons) and winds aloft (for fuel planning purposes). Pilot reports of weather conditions (PIREPs) may be delivered to ATC or Flight Service for local and long-range dissemination. This information helps other pilots avoid dangerous conditions and helps weather forecasters improve their forecasting models. The 7110.65 2–6–2 PIREP Solicitation includes a note:

Routine PIREPs indicating a lack of forecasted weather conditions, for example, a lack of icing or turbulence, are also valuable to aviation weather forecasters and pilots. This is especially true when adverse conditions are expected or forecasted but do not develop or no longer exist.

Live or periodic streaming of sensor information already present on larger airliners (particularly icing status, outside air temperature, and wind velocity, but other sensors could be installed as well) would relieve pilots and controllers of having to discuss routine PIREPs on frequency and could be developed into graphical systems that automatically display hazards. Delta is using vertical accelerometers in some of their aircraft to automatically report turbulence in real time and transmit that information to airborne flights; obviously they see this as a business advantage and are loath to share it, but the system could be expanded.
Whether the costs and complexities of such a system would be worth the real-time and forecasting benefits, I don't know.

"Streaming" implies a constant flow of data in a way that the available bandwidth of most present aircraft data/communication systems doesn't support.  It isn't inconceivable to route those sorts of sensors to an aircraft internet connection, but the present state of things is much more based on ACARS -- which is essentially a text message.  Sending a message every 10 minutes with winds aloft and temperature is entirely adequate resolution for the weather prediction systems, anyway.
It would certainly be possible to theorize a real-time wind readout from each aircraft to the controller so that he'd know the effect that wind would have when he gives a vector or a speed adjustment to an aircraft, but the value to be gained isn't all that great.  If every aircraft on a given arrival flow has been assigned the same heading & indicated airspeed, they'll all end up doing the same thing, even if the controller is having to guesstimate the winds based on the delta to their ground track from his assigned heading.  (Speeds never match anyway: assignments are given in indicated airspeed, but observed in groundspeed, which is true airspeed +/- the tail/headwind component.)
There is a case where it would be nice to have the wind values from a preceding aircraft, and that is in descent planning -- knowing the actual winds from cruise altitude down to 10,000' would allow a sufficiently smart FMC to build a more efficient idle descent path.  Nice to have, but probably not such an improvement over the current state of forecast winds as to make a big technology investment worthwhile.
Likewise, knowing what the winds are at various altitudes could better inform the decision when & if to climb to a higher cruise altitude.  Today, it's a matter of having a preflight weather forecast, and asking ATC if they have any aircraft at the other altitude that can provide a PIREP of winds and ride.  Again, nice-to-have, but only that.

Icing reports are quite useful in the us for general aviation.

US weather forecasts generated by the National Weather Service are generated by a supercomputer running the US weather model in DC. That forecast is sent to National Weather Service stations across the country and compared with local observations. Corrections are fed back into the model for the next forecast. The NWS also releases radiosonde balloons to gather data for the model. A third source of data for the model are collected by airliners.
Modern transport aircraft can now constantly transmit data to the airlines operating them. There is a world wide network of VHF ground receiving stations and extensive SATCOM coverage for data and voice. On board communication management units use nav data to auto tune for the region they are in and select the most efficient com means for the data depending on nature and priority of the data.  The real limitation is what an airline is willing to pay for.  Some airlines use onboard sensors to collect weather data and they have a contract to sell it to the NWS, presumedly for use in the model.
There are thousands of aircraft operating in US airspace at any given time. I don't know how many are sending data, but it's an excellent way to obtain data across the country at any number of altitudes.

Can you delineate what parts of this answer are "I believe" and what parts have a verifiable factual basis? Citing links that support specific facts helps elevate the quality of a post above the not-so-helpful "somebody on the internet said..." level.

A couple years ago I attended a 3 day pilots weather seminar and took a tour of our local weather office.  This is what they explained to me. I fly and look at aviation weather as a hobby. I'm sure anyone really curious could look at sources like this one or just go on FaceBook and ask the National Weather Service. They will answer.  https://www.ncdc.noaa.gov/data-access/model-data/model-datasets/numerical-weather-prediction


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What are the differences between a VOR and a VORTAC?
In Brazil airspace, we do not have VORTACs to navigate (unfortunately). That's why my doubts about accuracy, the range, the possible systems/frequencies they operate, and if there is any restriction to use it to navigate.

Just to be clear: you're asking the differences between a VOR and a VORTAC (that is, a type of beacon that's both a VOR/DME and a TACAN), or you're asking about the differences between VORs (or VOR/DME) and TACANs in terms of range, accuracy, etc.?

VOR stands for VHF Omnidirectional Range. It is a navigation beacon intended  for civil use and provides a user with a radial to/from the station. It works on frequencies between 108.00 and 117.95 MHz.
VOR/DME station PEK (Beijing):

source: Wikimedia, author: Yaoleilei
TACAN stands for TACtical Air Navigation, a military system that is similar to VOR but with higher accuracy. It works on frequencies between 960 and 1215 MHz.
Part of the TACAN is DME (Distance Measurement Equipment), which works in the same frequency band. The DME used in TACAN is the same that can be used by civil aircraft. Due to the higher frequency the system is more accurate and the antenna is much smaller and therefor easier to deploy (e.g. on a ship, on a battlefield).
 (source: wikimedia, US Air Force, Camera Operator: SENIOR AIRMAN JONNA N. WILSON)
A VORTAC combines the VOR and TACAN in one location. Civil users will use the VOR signals which have the same performance as ordinary VOR signals. In addition they use the DME from the TACAN. Effectively a VORTAC is like a VOR/DME.
Military users only use the TACAN, for them a VORTAC is the same as a TACAN.

source:Wikimedia, author: Hans-Peter Scholz Ulenspiegel


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Is there a source of deicing holdover time tables?
Depending on the types of de-ice and anti-ice fluids used, the HOT changes.
Is it possible to post some HOT table here?

I'm not sure what you are interested in. Are you really asking to have a list of all possible tables, and to have it here? If yes, I'm afraid this is too broad for this site.

Here are some holdover charts I have access to.

This FAA document has a lot of them, starting on page 11.
One example, from Transport Canada:

Are those times in minutes and seconds? i.e. for "Rain on cold soaked wing" in 27°F weather, the HOT is 10-50 seconds?

No, hours:minutes.

That makes more sense, but then it means that at the same temp, one de-icing is good for 12 hours against frost? That seems pretty amazing!


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What are the rules for right-of-way on final approach: by aircraft category or only by height?
I'm a student pilot studying right-of-way and confused about the landing provisions. Right-of-way is determined first by aircraft category, and then by position. That makes sense.
But the last provision, on final, says the lower aircraft has the right away. Does that mean that the lower aircraft regardless of the category has the right-of-way, or does the lower aircraft only have the right-of-way when they are in the same category?
For instance, if a glider was on final but high, and a Cessna 172 was on final but lower/closer, who would have the right-of-way?

The C172 has right of way, it's lower. The rules are in 14 CFR 91.113 and the only place where categories are mentioned is in 91.113(d), which is specifically about converging traffic at "approximately the same altitude". For landing traffic, the lower aircraft has the right of way.
People spend lots of time and energy discussing who has right of way on final, what "on final approach" really means, if it's OK to cut off a faster (or slower) aircraft, if an aircraft on a 5-mile final prevents other aircraft in the pattern from landing etc. But the discussion often becomes a bit academic: practically speaking the point is to be safe, not create any problems, and not cause other aircraft to go around or maneuver to avoid you. Think of it like pulling out of a side street in your car: if another car is coming then you shouldn't cause him to brake hard or swerve to avoid you.

Also if there is enough usage of a airport to get  many comfits, there will be a control tower.


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Why is the numerically determined location of the center of pressure inaccurate near Cl = 0?
I am working on an assignment and I need to calculate the center of pressure of an airfoil, through a numerical method (the panel method). I am getting a weird result because it gives its position on a point outside the chord for a determined angle of attack.
When I asked my professor he said that under an angle of attack near the angle of attack that makes Cl=0, it may come out that way, because the center of pressure is calculated through an expression that involves a quotient with the Cl=0 on its denominator.
I am confused because I don't know if it is just that the method fails in that case, or if it has some physical explanation.

Sounds like a good opportunity for further discussion with the professor. Since you're already paying him (indirectly through tuition), might as well get your money's worth.

sounds numerical, if you divide by 0 usually bad things(TM) happen.

Just a very, very general point about modelling and engineering: sometimes there is no "physical explanation", the model in not in the real physical world. You now have a great opportunity to learn something about the limitations of the model, any assumptions implied, when it might be expected to fail etc.

Thanks for replying, I will try to clarify it with my professor, but I found this on wikipedia: "It is common for the center of pressure to be located on the body, but in fluid flows it is possible for the pressure field to exert a moment on the body of such magnitude that the center of pressure is located outside the body."  So isn't it a nonsense to think it to be out of the chord ?

If some parts of the airfoil create lift and others a downforce, the center of pressure can be outside of the airfoil's chord at low lift coefficients. This condition is fulfilled for cambered airfoils, airfoils with a deflected flap and especially for rear loading airfoils, which have low camber in the front and high camber in the rear part. Supercritical airfoils meet this last condition.
In airfoil theory, an airfoil creates lift and a moment, because the lift is assumed to attack at the quarter chord point. This point is special because here the moment coefficient does not change with the angle of attack (at least in the inviscid, linear potential theory, which is sufficiently close to reality at large Reynolds numbers to be useful). In reality, positive camber will cause the resulting lift force to act aft of the quarter chord point, and the pitching moment is negative. When lift becomes small and the pitching moment stays constant, the lever arm of that small lift needs to become large to achieve the same moment, and this is when the center of pressure can slip out of the airfoil's chord.
Below you see the result from XFRL5 V6.0.5. I plotted the local center of pressure as a green line. It's elevation over the plane of the wing shows the amount of lift locally created, and the location in streamwise direction shows that it leaves the local chord when lift becomes low. Note that when moving out in spanwise direction the location jumps from far aft of the airfoil to far ahead when the local lift turns negative. At the point of no local lift you have a division by zero error, patched over here by a straight connection between the results from the single panels.

Location of the local center of pressure on a swept wing (own work).
When angle of attack is increased, all additional lift has the Birnbaum distribution, so the local center of pressure moves towards the quarter-chord location.

Could you elaborate a bit on the Birnbaum distribution? I tried to Google it, but I could only find references to unsteady flapping aerodynamics such as this one or should I ask this in a new question?

@ROIMaison: I trust you understand German, so I recommend this page.

Thank you for such a complete explanation. It still sounds me a bit counterintuitive. Mathematically I see it, when you weight the position "x" with the local lift coefficient (to calculate where the resultant is applied), since both "x" and "cl" can be negative, there may be some contribution that tends to set it out of the chord. Am I missing any important idea or simply there is no more on it?

@abcd The airfoil is creating both a moment (measured around the quarter chord point) and lift, and the moment is determined by camber, so it does not change with angle of attack (linear theory). When lift is very small, the lever arm to express the moment must become huge.

So, would I be right in saying that the main problem here is that "centre of pressure" is an aggregate function, rather than a simple force? The wing itself is only providing lift "within itself" but that CoP also includes other components?


text:
What is the physical meaning of Mean Aerodynamic Chord?
I have not been able to find a consistent definition of what it really is.
I have come across definitions such as: "Physically, MAC is the chord of a rectangular wing, which has the same area, full aerodynamic force and position of the center of pressure at a given angle of attack as the given wing has." which is contradictory with this one "The mean aerodynamic chord is (loosely) the chord of a rectangular wing with the span, (not area) that has the same aerodynamic properties with regarding the pitch-moment characteristics as the original wing."
I am very confused and I wonder if you could please give me a precise definition (maybe in relation to an equivalent rectangular wing, as those two definitions I found), that shows intuitively what it is

The mean aerodynamic chord is only identical to the mean chord for rectangular wings. For tapered wings it is slightly longer.
Why? Because the mean aerodynamic chord is the mean chord of a rectangular wing that has the same pitch characteristics as the "real" wing. Pitch characteristics need to include pitch damping, and pitch damping grows with the square of the local chord. That is why the formula for the mean aerodynamic chord divides the square of the local chord by the wing area:
$$\text{MAC} = \int_{y=-\frac{b}{2}}^{y=\frac{b}{2}}{\frac{c^2}{S}}dy$$
Here $b$ denotes wing span, $y$ the spanwise coordinate and $S$ the wing area. Since the chord is squared, deeper sections of the wing are overrepresented in the result. The resulting rectangular wing will have a larger area than the original, tapered wing but the same pitch damping! In case of a delta wing, MAC will grow to be ⅔ of the root chord, and for an elliptical wing it will be 90.5% of the root chord.
The MAC has been invented to convert arbitrary wing planforms into much easier to calculate rectangular wings. By doing all calculations on the correctly sized rectangular version, the more complicated calculations on the real one could be avoided. However, this works only up to a point:

if you want to calculate lift, you need a wing of equal area.
if you want to calculate induced drag, you need a wing of equal span.
if you wand to calculate pitch motion, you need a wing of the correct MAC. This rectangular wing can either have the same area or the same span as the real wing, but not both together, unless the real wing is rectangular, too.

You are right, the two definitions you quote are not compatible. The rectangular wing cannot have the same span than the real one, or its area and all associated forces and moments would be higher.

Thank you very much for such a clear answer. I still have a question on why wikipedia says: "The pressure distribution over the entire wing can be reduced to a single lift force on and a moment around the aerodynamic center of the MAC". My question is, can't a pressure distribution always be reduced to a single force and a moment in any point? Or is there something particular there? And finally, what you said imply that the first definition I mentioned is wrong or they are compatible? Thank you again

@abcd: Yes, you can always collect all pressure as a force in a point plus a moment. See here for a longer explanation.

On transport aircraft, as you have already gathered the position of its centre of gravity is expressed in relation to the MAC. The definition of MAC, that I will provide is I hope a little clearer. If you were to take a plan view of the wing and draw any number of chord lines over the surface you would notice they're of various lengths, usually longer at the end closest the fuselage (wing root) and shorter at the end farthest the fuselage (wing tip) with different distances from the nose of the aircraft, the shortest distance being again at the root and farthest at the tip.
Applying mathematics to find the mean of all those chord lines you have the MAC.
This is shown as a single length beginning at the 'reference datum'.
For example, an aircrafts MAC may be 520cm in length extending from 2006cm to 2526cm aft of the reference datum.

Mean aerodynamic chord is the chord drawn through the centroid (geographical center ) of the plan area.remember it is not the average chord but it ia the chord through the centroid of the wing area..


text:
Do some aircraft lower the landing gear mid-flight?
Are there situations in which a pilot could/has to lower the landing gear during flight for reasons not directly connected to landing? (i.e., not part of the landing checklist)
Originally I thought I had seen some around the location indicated by the pin on the map below (17° 25' 30.093" N,  78° 32' 27.4734" E), in the evening time around 4:00  and the weather was clear.

See http://meta.aviation.stackexchange.com/questions/377/why-is-a-question-force-edited-to-meet-the-answer-given-below-but-not-the-questi

That's nowhere near midflight. You are seeing aircrafts approaching Begumpet Airport (VOHY)'s runway 27. This airport has no airline service, but it is used for some general aviation including VIP flights and as military airbase.
The point directly north (about 3 km) from the coordinates you gave (N17° 25' 30", E78° 32' 27") is about 6 km, or 3.2 nm, from threshold of VOHY runway 27. Given usual 3° glide slope the aircraft would be just about 1000 ft above runway elevation, which happens to be the latest point where landing checklist should be complete. Yes, that means gear down. It also means it will be touching down in about a minute and a half.

@user285oo6: http://en.wikipedia.org/wiki/VOHY: Begumpet is now used for military aviation training and for flights carrying VIPs. Also http://ourairports.com/airports/VOHY/: Begumpet is still used for some private aviation and as a military air base.

The reasons I can think of offhand for extending the gear for other than landing are:

As has been mentioned in a previous answer, use of the landing gear as a speed brake. The Fokker F-27 had a speed brake switch on the front panel that when used lowered the main landing gear but left the nose gear retracted. At the two 747 carriers I worked for in the 1990s, we occasionally dropped the gear to get more drag for a steeper descent. As I remember the max gear extension/retraction speed was 270 knots, but once extended the max speed was 320 knots.
To cool hot brakes or hot tires. This was the most common reason in my experience for extending the gear. The 747-100/200 was supposed to be taxied at no greater than 25 kts, which meant that braking was needed during taxi even with engines at idle. Tire flexing also produced significant heat. There was a recommendation not to taxi more than 30,000 feet as I remember because of these factors. If you were using, say, the reef runway at Honolulu, you exceeded that distance by the time you lifted off, and if you were above 800,000 lbs you were guaranteed serious heating.
To pre-cool the brakes and tires if you had a short leg followed by a quick turn. The 747 was originally designed to stay in the air for awhile before landing. The story I most often heard was they designed for 5.5 hours of cooling in the wheel wells before the gear would be used.
To burn extra fuel to get below landing weight. Rare, but it could happen if you were tankering fuel such to arrive at max landing weight and for whatever reason the leg burn was less than planned. Fuel burn increase with the landing gear down is quite dramatic. We were tankering fuel once from Yakota, Japan to Osan, Korea, and we couldn't get the landing gear up. Our choices were to dump fuel and return to Yakota or to proceed to Osan with the gear down. We chose the latter. We couldn't get above 25,000 and our fuel burn was twice normal. Confused the controllers; they kept wondering by we were so slow and so low. We kept having to explain.

Yes. If you want to increase drag to lose altitude quickly. For example, mountain flying, need to descend rapidly and speed brakes + flaps just aren't cutting it.
Fun fact: The F4U Corsair used its landing gear as speed brakes. There was a special way they lowered them in this case. (I belive the hydraulic struts weren't locked ect., so landing on them would not be smart)

Common during emergency descent procedures.  Get down to gear & flap operating range, drop them, tight spiral dive at the highest speed permissible in that configuration.

Military uses this technique when needing to to land at a front line airfield and straying too far from the field will be dangerous.

Landing gears are generally retracted to decrease drag in flight . As the speed of
an aircraft increases, so does parasite drag. Mechanisms to retract and stow the landing gear to eliminate parasite drag add weight to the aircraft.
Private Jets generally use  Tricycle-type landing gear with dual main wheels . One of the main advantages of using the Tricycle-type landing gear is that it prevents ground-looping of the aircraft. Since the aircraft center of gravity is forward of the main gear, forces acting on the center of gravity tend to keep the aircraft moving forward rather than looping, such as with a tail wheel-type landing gear.
It is not necessary to retract the landing gear . Infact there are Landing gears that are Fixed as can be seen in many small, single engine , light aircrafts .
On certain occasions Private jets as well as large aircrafts like the Boeing 777 extend their landing gear during flight to cool the Brakes because :

The brakes may have been heated by the previous landing and not had time to cool while the airplane was at the gate.
During taxi out, they were further heated by necessary brake applications.
After takeoff, the landing gear retracted into the wheel well where there is limited cooling air.

Do you have a reference (e.g. an operations manual) for extending the gear during flight for this purpose?  Many aircraft have speed restrictions for extending or retracting the gear that would make this unfeasible during cruise.

@ casey - I have a couple of Aircraft Operation Manual . They are all in paperback . The only Manual in pdf format is this http://www.deltava.org/library/B777%20Manual.pdf

That "manual" is produced by flight sim enthusiasts and doesn't address the in-flight gear extension.  If one of your manuals does address it, I'm willing to bet it is in the QRH under abnormal/emergency procedures.  In bullet points #1 and #2 of your list, an airplane would delay its takeoff if the brakes were too hot to ensure adequate performance to abort the takeoff at V1.  I'm still quite skeptical that outside of an in-flight emergency (brake overtemp/fire, emergency descent, etc) that anyone is dropping the gear during cruise.

After takeoff, the landing gear retracts into the wheel well where cooling air is limited . This will cause the brake hot light to illuminate the sensor and generate an Electronic Centralized Aircraft Monitoring (ECAM) alert if the temperature exceeded 300?C. Consequently,the crew would follow the ECAM procedure and extend the landing gear, allowing the air to cool the brakes.

@D_S: Therefore it is nowhere near cruise but only during climb-out.

@d_s paperback manual can be photographed?

I am on a U.S. Air flight right now.  Pretty sure that the plane is an A330.  About 20 minutes after take-off, the pilot said that he had to lower the landing gear to cool off the brakes.  After about 3 minutes of much noise, and some chop, he raised them and said that it had worked.

That is quite out of the ordinary, but it happens sometimes when turn around times are short or there was an aborted take-off. Heat dissipation from the brakes can take quite some time so it normally the measured maximum brake temperature occurs quite a while after the brakes were applied. Usually under such circumstances the gear is not retracted immediately after take-off, but what you describe is not unheard of.

And welcome to Aviation.SE.

I cannot imagine much need to lower the gear during flight, except perhaps if there was an issue.  Also doing so mid flight would required restricting speed to the lower of Vlo (landing gear operating speed) or Vle (landing gear extended speed).
The actions that one takes that involve dropping the gear are usually part of some error handling procedure.  These may be documented in operational manual or be something defined during training on the specific aircraft.
The one time I dropped gear in flight it was after one of the gear lights failed to turn off (indicating gear up and stowed) after retract.  I slowed up to stay within the operational limits, dropped the gear and then retracted the gear.  This time the gear light stayed off.  After landing I checked the gear switches for a stuck piston or loose wire.  I did not find anything and the problem never repeated.
If the problem had been something like a gear motor circuit breaker popping I would have reset the breaker ONCE.  If the breaker popped again I would have cranked the gear down and left it down.

Sure it's a private jet? Plenty of single-engine piston powered aircraft have fixed landing gear.
Most jets cruise at high altitudes, so if you can a) see the aircraft, b) identify it as non-commercial, c) notice the landing gear is down, and d) claim that it's not near an airfield then it's more likely that it is a 4-6 seat piston single with fixed gear. Cessna, Piper, Beechcraft have made thousands.
Addendum to edited question:
You say there are no airports near by. Possible there's one a little further away? Planes typically raise the gear as soon as they are off the runway, but they lower it 30km out. And that's 30km flying distance, not driving diatance.

a)It's a commercial jet with twin engine plane(a kingfisher plane) b)its a medium sized plane not too small(a trainer craft with prop in the centre) or not too big (Airbus A380).

You missed the other option: that it is nowhere near midflight.

@JanHudec yes, his statement that it's "clearly visible from 30 feet" made me frown at the "mid flight" bit.

@jwenting what made you frown a bit

@user285oo6 the claim that he saw it 30 feet up yet it was in mid flight...

@jwenting The claim is that he saw it while standing on a terrace 30ft above ground level, not that the plane was 30ft AGL.

@paul this is not a Cessna I guess and sure its not a single seater [plane](http://www.google.co.in/imgres?imgurl=http%3A%2F%2Fcdn-www.airliners.net%2Faviation-photos%2Fphotos%2F8%2F3%2F2%2F1866238.jpg&imgrefurl=http%3A%2F%2Fwww.airliners.net%2Fphoto%2FKingfisher-(United-Breweries%2FBoeing-727-44(RE)-Super%2F1866238%2FL%2F&h=695&w=1024&tbnid=XqXx1IUMKPQDiM%3A&zoom=1&docid=QpCBXl7DMpiulM&ei=oCLfU-zYK9WgugSnzoHIBQ&tbm=isch&ved=0CF4QMyg5MDk&iact=rc&uact=3&dur=1010&page=3&start=45&ndsp=21)


text:
How is engine thrust measured in flight?
When aircraft engines are evaluated in a test flight, how is the thrust of the engine measured?
You would need accurate thrust numbers to calculate the real-world specific fuel consumption (i.e. not in a controlled environment like an engine on a test bed).
I'd assume it is nearly impossible to measure thrust indirectly with any accuracy since so many factors are involved (wing shape, wind, air density, lift, ...).
So I guess there should be a way to measure thrust directly, probably in the engine pylon.
So how is engine thrust measured in flight?

Specific fuel consumption is property of the engine (and propeller if applicable) assembly only. Those can be just put in a wind tunnel or on a test rig!

My question concerned the specific circumstances of a real test flight, including wind, temperature, air pressure, moisture, ...

I don't think it's actually interesting value. You need to know consumption at given speed under given conditions, which can all be measured directly and the thrust is mostly irrelevant intermediate value at that point. It is really only useful for comparing engine performances when choosing engine for an aircraft and for that purpose measurement on a test bench is plenty good.

You would still need to measure thrust to detect anomalies.  If your engine uses the right amount of fuel but produces less thrust than expected, that's a very dangerous situation.

It is unclear if your question only address engine based on turbine (trubojet, turbofan, ...) or also include other engines (piston, electric, rocket, ...)

@florisla - if the "normal" airspeed for a given throttle setting and fuel burn rate isn't being met, that's an anomaly. A given airspeed is very simply the speed at which thrust is cancelled out by drag, so it's as good an indicator as any of a potential problem.

@KeithS But then you still don't know if the anomaly is caused by abnormal drag or by abnormal thrust.

Engine thrust is measured in flight by EPR - Engine Pressure Ratio.
EPR is the ratio of the turbine exhaust pressure divided by the pressure measured at the fan or inlet. Indeed this is the measure used for a number of engines for setting thrust.
More detailed airborne testing of engines is done during development, most manufacturers have airborne testbeds. The parameters recorded here probably measure in the hundreds or even thousands...

Not all airplanes have EPR.  For instance, my jet engines only have N1%, N2% and ITT (well, and fuel flow, oil pressure/temperature, vibration meters, etc. too)....

Sure, but EPR is the closest thing to an airborne measure of thrust, whereas N1 is more a measure of power set (percentage fan speed).

Absolutely, but your answer does not address how thrust is measured in an aircraft without EPR gauges....

I suppose it isn't directly. Its the same as Peter's answer on piston engines. Whilst in piston aircraft you generally set RPM or Manifold Pressure. In Jet aircraft setting a percentage N1, is setting a percentage of the available thrust, albeit predetermined and not measured "live".

VectorVictor gives the right answer for jets.
However, for propeller aircraft this is really hard, and in the end only the difference between drag and thrust can be measured. By measuring the brake power of the static engine and the drag of the aircraft in a windtunnel, some data points can be acquired which help in calculating what the real thrust in flight could have been.
You are right, in the end it is impossible to produce a precise measurement of thrust. The most important part of the measurement is actually a precise definition of what thrust is: How do you account for the increased friction drag in the propeller slipstream? Is cooling drag part of the airframe drag, or should it reduce thrust? Measuring thrust is foremost an exercise in precise, meticulous bookkeeping.

For simple platforms, load-cells can be attached in between the propulsion system and the airframe. This allows the propulsion force to be measured.
This force data excludes aerodynamic integration effects, such as:

propeller stream effects for nose-propped aircraft, and fuselage blockage for rear-propped aircraft
engine cooling drag (which is an aerodynamic property of the particular engine/propulsion integration)
accelerated air over the airframe (in-body jet engines, or propeller aircraft)

This is not an absolute requirement to assess the performance of an engine. Especially, if the aircraft can fly with another engine or if the drag characteristics of the aircraft is known by some valid method, instead of a direct measurement, indirect calculations can be done. For example,

for constant speed and level flight, thrust = drag
for accelerated level flight F_net=Mass * acceleration, etc.

A new or overhauled engine's thrust on a manufacturer's test bed is measured against a load cell or thrust meter at given RPM, TGT (Turbine Gas Temperature),fuel flow and EPR (Engine Pressure Ratio).
When the certified engine is installed in the airframe and ground run, after taking into account local atmospheric conditions,intake and jet pipe losses, the maximum EPR, or thrust, and TGT and fuel flows can be cross checked.
In a test flight, given the correction factors for altitude and speed etc, against the known parameters of EPR, TGT, RPM and fuel flow, the engine can be seen to be giving required performance. If the the engine parameters are correct but aircraft performance (i.e. speed and rate of climb) are low then suspect the airplane's weight and/or drag (flap, doors, panels etc poor fit) as possible culprits. Its not fair to expect an engine to push a dirty heavy airplane up that big hill on a hot day is it?


text:
Is the Progress D-27 really a propfan?
On WikiPedia, the Progress D-27 is described as a propfan engine and the airplane it powers, the Antonov An-70, is described as propfan-powered.
This Air&Space magazine article contradicts that; it states that the An-70 is turboprop (not propfan) powered, as is the Airbus A400M.
And indeed the basic configuration of the D-27 and the A400M's
EuroProp TP400 turboprop are very similar:

4.5 m propeller diameter vs 5.2 m
14,000 hp vs 11,000 hp
two-shaft gas generator, third shaft to drive propeller
propeller driven through reduction gearbox
propeller blades mounted before engine
780 km/h max speed vs 780 km/h cruise speed
variabele pitch blades

The only significant difference is that the D-27 has two contra-rotating blade sets while the TP400 uses only one blade set.
Having contra-rotating propellers does not necessarily make a turboprop a propfan though (e.g. the Tu-95 is a turboprop aircraft).
So all things considered, what would make the  D-27 really a propfan but not the TP400?  Is there really a distinction between a propfan and a turboprop?

It depends
It seems that currently the term is more a marketing term than an engineering term.

When fears arose among airline officials that the word “turboprop” would meet with consumer resistance, the term “prop-fan” was used in a poll of United Airlines passengers. It worked: 50 percent of the respondents said they’d fly on a prop-fan-powered airliner.

From Air&Space Magazine

It depends which particular features you consider an essential defining element for the term "propfan".
Turboprop?
Many features of propfans are also found on some powerplants described as turboprops by their makers.

Contra-rotating blades? - Found on, for example the 1950's NK-12 turboprop.
Supersonic tip speeds? - NK-12 turboprop again.
More than four blades?

The 1945 RB.50 tuboprop had five.
The 1984 PW127 turboprop drove a six-bladed prop.
The 2009 TP400 turboprop drives an eight-bladed propeller.

Curved blades? - TP400 turboprop.

Gearbox?

No Reduction Gearbox - A key distinction of a key forerunner, the GE36, was the lack of a reduction gearbox as used in turboprops.

Other names?

GE called their GE36 an unducted fan (UDF).
Rolls Royce call their design an open rotor engine.
The acronym CROR is also in used; it stands for Contra-Rotating Open Rotor.

D27?
The marketing department of its designers call it a propfan.

Dictionaries
Oxford says

Turbofan: A jet engine in which a turbine-driven fan provides additional thrust.
Turboprop: A jet engine in which a turbine is used to drive a propeller.
Propfan: No results found for “propfan”.

Mirriam Webster - same as Oxford essentially.
Aviation terms.com also defines turboprop etc but not propfan.
Aviation Canada, and other aviation glossaries I looked at, also fail to mention propfan.
Dictionary.com says

propfan. Aeronautics. a turbojet having a turbine-driven propeller that operates completely outside the jet engine.

But that definition could equally apply to any turboprop.
Wordsense.eu says

propfan (pl. propfans)
  A modified turbofan engine with the propeller outside the engine duct of an aircraft.

Which is not any better.
Wikipedia tries to define propfan in terms of what it isn't. The nearest it gets to a definition is to say that a propfan has a propellor with many short twisted blades. However the TP400 has many twisted blades - perhaps they are not short enough to qualify?
Conclusion?
There doesn't currently seem to be a clear and short widely-agreed or widely-used set of engineering criteria for labeling a particular engine a propfan rather than an advanced turboprop, an unducted turbofan, an open rotor engine or any other name.
The D27 is a propfan because its maker's marketing department calls it that.

I would really like to give you +1, but you keep misspelling its :P

I already suspected marketing was playing a big role in naming a turboprop a propfan.  "open rotor" is a commonly found name, as is CROR - contra rotating open rotor.

@Federico: Aargh!

@RedGrittyBrick Do you have a source for your conclusion that there is no clear distinction? I'd look in WikiPedia but the article there states that a propfan is distinct from both a turboprop and a turbofan.

@florisla: I can't give you a single source. I have found that there are many contradictory definitions and that mainstream traditional dictionaries have no definitions.

Yes, it is a Propfan Engine. This explains the increased number of blades in the propeller. Higher Disk Loading requires the use of smaller diameter propellers. The blades are scimitar-shaped, with swept-back leading edges at the blade tips to accommodate the large Mach numbers encountered by the propeller tip at high rotative and flight speeds.
http://ivchenko-progress.com/?portfolio=d-27&lang=en
In fact the manufacturers of AN-70, SE Ivchenko Progress, themselves describes the D-27 as a Propfan (also here
EDIT :

A propfan consists of swept, rotatable airfoil blades pivotally
  mounted to a hub for pitch change movement with respect thereto, and
  having a solidity ratio of 1.0 or greater at the roots of the blades
  and less than 1.0 at the tips of the blades. The prop fan is operable
  at or above critical Mach numbers and at transonic or supersonic tip
  speeds, and is characterized by: each of the blades having a leading
  edge. The leading edge, from a location thereon at approximately a
  midportion of the span and the blade, outwardly to the tip thereof, is
  curved in a chordal direction to define blade sweep while exhibiting
  no significant offset curvature in a span-wise direction.

Source: http://www.google.com/patents/US4730985

Just because the marketing department calls it a propfan does not make it true; I'm not convinced yet.  The TP400 has only one blade set so its disc loading is higher than that of the D-27.  It also has scimitar-shaped blades.  Its speed is higher (cruises at 780 km/h which is the An-70's maximum speed).

I have edited my comment to include what type of engines are defined as PropFan. It's not just a marketing department classification , tests were conducted by the country's Ministry Of Defense to test the performance of the SV-27 propfan intended for the D-27 http://www.ruaviation.com/news/2013/4/12/1630/

Van you clarify why the tp400 is not a propfan? It also has variable pitch blades although their shape definitely differs from the D-27's.

Yes. It's a prop-fan. However, it's not trying to give it a name that's important. It's knowing what it does and how it does it that is important.
So, this is what it does: "Propfan, so called, because it combines the operating efficiency of the turboprop with the higher speed and altitude capabilities of the turbofan." ref Hamilton Standard/ NASA CR 174790. This can be achieved in a variety of ways: single rotation or counter rotation, wing or fuselage mounted, tractor or pusher, geared or gearless. ref "Single rotation and counter rotation prop-fan propulsion system technologies" ICAS-84-5.6.2. So, whatever particular configuration is chosen to bridge the gap between the conventional turboprop and turbofan then it's safe to call it a prop-fan, but only if you're talking to someone with the same understanding.

An unducted fan or propfan is defined as a fan or fans being directly driven by a set of turbines, that is, no reduction gearbox. The unducted fan is essentially a turbofan with the fan being outside the nacelle, hence the term "unducted." Early unducted fans were directly integrated onto the turbine section of the plant resulting in the fans being in the aft of the engine, however if you run a shaft to the front, as is done on a turbofan, it can be a tractor.
A turboprop is defined as having a gearbox to connect the turbine section with the propeller.

Where is it defined like that?  And is that definiton generally accepted?


text:
What is the busiest route of Turkish Airlines in 2017?
What is the busiest non-domestic route of Turkish Airlines:

in terms of passenger number,
in terms of number of flights performed per year?

Is there a database that I can look for such kind of statistics for all other European airlines?

I'm voting to close this question as off-topic because it would be better suited for travel.SE

@Pondlife As a regular user of travel.SE, I'm not at all convinced that this question is on-topic there. They'd likely bounce it straight back here saying that it's a question about the airline industry, not about travelling. (As a general rule of thumb, ask yourself, "Would any traveller need to know this to make their trip?" I think the answer is clearly not. A traveller might need to know how busy a particular route was, to know how far in advance they should buy a ticket. But it would never be useful to know an airline's busiest route.)

@Pondlife further to what David said, the fact that it's potentially on topic elsewhere isn't a good reason to say it's off topic here. This is very much about aviation in my opinion.

@DavidRicherby That's fair enough, I'm not very familiar with travel.SE's scope. This just doesn't feel much like aviation to me, but that's OK, that's just my opinion :-)

@Notts90 I disagree, but that's why SE has votes :-) If most people feel it's on topic that's fine.

@Pondlife I think the question fits to aviation title quite well. What is "aviation" then? We should not narrow-down our minds. I am sorry that the question was shot-down by 5 members. Is that possible to make a voting whether the question on-topic?

You can check the help center for what's on topic here but like all SE sites there are always grey areas, and ultimately we consider each question individually. Right now, people have already voted that it's off-topic but you can always edit the question to make it more relevant, or you can ask in [chat] for people to reconsider and vote to re-open. For me, the question has very little to do with operating aircraft and that's why I voted to close. Others may feel - and vote - differently, and that's completely OK.

@Pondlife Pardon me! Thanks for the detailed answer. Of course, everyone can have different opinions. However, does aviation mean only operating an aircraft? Why should not it include operating an aircraft (related) business?

According to Turkish Airlines VP Ziya Taskent, they are adding a ninth weekly flight between Tel Aviv and Istanbul. He mentioned that:

695,000 [passengers] flew roundtrip with the airline in 2014 [between
  Tel Aviv and Istanbul], 4.86% more than in 2013, making it the leading
  foreign airline in Israel

The largest number of airports served by the carrier is Germany with 14 destinations.
For other European carriers as requested, the Wikipedia article 'List of busiest passenger air routes' is a good starting point.

This is extremely interesting to me. Last week, I randomly came across a flight path (Turkish Airlines) between Istanbul and Amman (Jordan). I noticed that they clearly explicitly fly around Israel to avoid it. I thought that the airline may have had diplomatic issues with Israel but I guess not. Any guesses why they do that? Here: https://flightaware.com/live/flight/THY812/history/20180125/1835Z/LTBA/OJAI

@SamCreamer:  I'd encourage you to ask that as a new question.  Interestingly, the return flight flies directly over Israel:  https://flightaware.com/live/flight/THY813/history/20180126/0355Z/OJAI/LTBA

@MichaelSeifert That is very interesting... That leads me to believe that there are some restrictions on Jordan's end. I will ask this as a new question so that others can have more visibility in case they are curious.

@MichaelSeifert I have asked a new question here: https://aviation.stackexchange.com/questions/48003/thy812-explicitly-flies-around-israel-but-return-flight-thy813-flies-right-ov


text:
What is this crossbow-shaped flight path?
While headed to lunch yesterday I spotted a small aircraft flying low overhead. I didn't think too much of it until a few minutes later the exact same plane flew over again in the exact same spot! I pulled up a flight tracker on my phone and saw that it was flying in the following strange pattern:

Link to the flight path on ADS-B Exchange
The flight path consists of two back-and-forth passes in an East-West arc, six loops of a North-South oval, and two larger and more rectangular loops that end over Andrews Air Force Base. I'm struck by how precise the flying seems. What's more, looking up the flight path later it looks like the plane flew the same pattern in a different location later that same day, just rotated in a different orientation:

What is the purpose of this strange flight path?

Does this answer your question? What are flight check operations? and see also the question already linked by OP: What is flight inspection?

After doing some research I believe this aircraft was inspecting or calibrating the ILS (Instrument Landing System) at the two airfields it flew over: Andrews AFB (ADW) and Danbury Municipal Airport (DXR).
Compare the flight path shown in this answer to the question "What is flight inspection, as provided by Cobham Aviation Services?", which shows a similar path consisting of an arc and loops, described as "to inspect the performance of the Instrument Landing System."  Similarly, this YouTube video "FAA Beech 300 King Air performing CALIBRATION FLIGHT at Boston International" shows a nearly identical flight path.
Actually, the aircraft shown in that video, a Beech 300 King Air, is the exact same type of aircraft I saw:

I assumed it was a bug or missing data, but apparently the registration is actually only 2 digits, as also seen in the video. Even the "flight number" matches the video, and gives us a clue as to the flight's purpose: FLC for "flight check."

You have correctly answered your own question! This was a Flight Check aircraft making sure that the radio signal from the ILS behaved as expected. Not sure this question should stay open, however... may be a duplicate.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


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Can the MEL be equal to MMEL?
I am performing a review of aircraft maintenance. i intend to reconcile the MMEL for airbus 320 along with the operators MEL. Based on my information, the MEL is supposed to be more restrictive than the MMEL as it considers operational hours, operating conditions and many such factors. In this case, is it OK it the MMEL and the MEL have the same item list? Shouldn't the MEL have some more items?

If MEL had to have some more items, where would they come from if they were not on the MMEL?

It may matter if you are in the FAA world or someplace else, so you might want to clarify. In my experience the operator is free to add or remove items from their own MEL independent of the MMEL, as long as they have justification and FAA approval. The MMEL serves as a baseline just like the MPD for maintenance. Of course removing items from the MEL or making them more restrictive than the MMEL is easier to justify than adding new items or making items less restrictive.

@JanHudec One example would be if the operator did an STC, like wireless internet. That STC would not be in the MMEL, but there is no reason to ground the plane over the wireless being inop. So the operator would add the wireless system to their MEL list.

Check out FAA MMEL Policy Letter 34, Revision 4: http://fsims.faa.gov/PICResults.aspx?mode=Publication&doctype=MMEL%20Policy%20Letters it does a decent job of breaking down the difference in a MEL and MMEL.

Thanks for your answers guys! I checked both the lists and both the lists seem to be the same. The risk I feel is if the MMEL and MEL have exactly the same items may be the operator is not considering crucial factors like operating conditions and including some more items in its MEL

@OSUZorba, that's the other way from what I meant. And we are back to confusion whether we are speaking about list of items the aircraft is permitted to fly without, or items it is forbidden to fly without.

@janhudec the MEL and MMEL for commercial aircraft list items that can be inop and any restrictions of having those items inop, they do not list everything you must have to fly, despite the name. Check out the FAA policy link I added above. Even if it was the other way, an operator could add something via STC that is required, such as a glass cockpit. Here is the direct link to PL-34: http://fsims.faa.gov/PICDetail.aspx?docId=PL-034

@OSUZorba, make the comment an answer here so you can get some reputation for it ;-).

@AtishNair, please note that your question does not have any answers! So there is nothing to thank for!

@JanHudec: Can you provide me with an answer then?

Found certain items in the MMEL but not present in the MEL. What does that mean?

I am no authority on the topic, but I'll at least try to summarise the bits from comments:

MEL can contain additional items not on the MMEL only if:

They concern after-market modifications or additional equipment and the corresponding STC permits it. Typically this includes things like IFE, wifi-routers and similar items for passenger comfort, not necessary for safely conducting the flight.
Sometimes it might be possible to add a normally required item if additional equipment is installed that can work as a backup, subject to overseeing authority approval.

MEL can omit items from the MMEL if:

They concern optional equipment that is not installed on the particular aircraft.
The operator chooses to omit them. This can be done because the associated restrictions would not allow the plane to complete the typical mission profile with that operator anyway or if the operator considers the restrictions to be too much trouble for what they are worth (e.g. would require otherwise unused procedures that would have to be trained).

If all those categories happen to be empty, the MEL will be equal to MMEL.

The MMEL cannot just be used as the MEL.  It had to be approved by the FAA for your specific serial number... Hence just a MEL.


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Is it possible to become a commercial pilot if one suffered from depression in the past?
I am 33, from Canada, and am trying to become a commercial pilot. I secured some funding and signed up for a college program...Then I came across a few articles detailing how past mental health issues can bar you from being applicable to fly commercially. My question is; If I have had a period of depression in my past, is it an absolute, that I will not be able to pass my medical evaluation test? Is there a time component element to it?...ie..If a person has not shown signs of mental illness for a certain duration of time, can they pass the aviation medical?
Also, I have not been prescribed or taken any mood altering medication in over 3 years.
Cheers

This question isn't an exact duplicate, even if the answer is likely to be similar: this one is about Canada, whereas the other one is about the US. Medical regulations are different in each country.

The answer to this question is substantially identical to the duplicate: You need to consult an aviation medical examiner in your jurisdiction (Canada) to determine if your condition is disqualifying. The relevant section of Transport Canada's regulations include "psychiatric illness", which may include depression, but is not wholly clear on the criteria for issuing/denying a medical.

@Pondlife ah. good point, somehow overlooked that, sorry.

I really can't speak for the Canadian standards for Airmen Medical Certification, though I did find this on Transport Canada's website.
https://www.tc.gc.ca/eng/civilaviation/regserv/cars/part4-standards-t42402-1412.htm
Transport Canada also lists the following concerning anti-depressant medications (SSRIs).
https://www.tc.gc.ca/eng/civilaviation/opssvs/cam-ssri-347.htm
This sounds very similar to the FAA's position in that people who are diagnosed with depression requiring medication are prohibited from obtaining a medical certificate.
OK, don't throw yourself off a bridge just yet; there's some good news.
The good news is that this is applicable if you are currently on mood altering medications perscribed by a physician.  If you have a prior history of depression but are not currently on any medication, there is a good chance that Transport Canada could issue you a Special Issue medical certificate, if you are willing to go through all the extra paperwork and documentation with your physician.  You are going to have to document, probably with the written concurrence of your physician, that you are no longer depressed, your case history was acute, not chronic with recurring episodes, and that you no longer require medication to maintain a normal mood.  Transport Canada will probably issue you a Special Issuance Airman Medical Certificate, valid for about a year, which will allow you to obtain your license.  As long as you are not diagnosed with further episodes of depression during this period and your doctor can confirm this, TC will probably issue you a regular airman medical certificate thereafter.
What I would suggest is that you make an appointment with a physician near where you live who is a Designated Medical Examiner for Transport Canada and talk with them about what TC's disopositon is on disqualifying conditions for medical certification.  NOTE WELL: Do not - NOT - make the appointment so as to indicate that you want an official airman medical certificate examination; we do not want any kind of official diagnosis entered onto your permanent record with Transport Canada just yet; this will only complicate things and we need to know exactly what TC's position is on these issues BEFORE we proceed down that road.  Select a DME who is also a pilot and will tend to be more sympathetic to your plight.
I hope that helps and good luck to you on your future aviation endeavors.  There is hope for you.


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Why is wireless avionics intra-communications (WAIC) not used for air-air or air-ground communication?
Since ITU approved the use of WAIC systems, aircraft designs have evolved and benefited greatly from fuel consumption savings, reduced maintenance (by removing/replacing costly and heavy cable wiring with wireless systems) and improved environmental footprint. However, these wonderful lightweight WAIC systems are restricted to provide bounded inflight services and does not provide air-to-ground or air-to-air communications. What is the reason for this?

The short answer seems to be that the ITU says that WAIC is based on low-power, short-range transmitters and its main purpose is weight reduction. So it was never designed or intended for air-to-ground use. If you're thinking about streaming flight data off the aircraft, this question has more information.

@Pondlife. Thanks for the interesting post on streaming.

WAIC is designed to be extremely low power.
Its low power is not be enough to reliably reach a receiver on the ground, satellite, or other aircraft.  Increasing the power would create all sorts of other problems, including problems with spectrum allocation, and data collisions.
There is simply no demand for a more complex system to do these other tasks.
The basic engineering principle is KISS

sorry i was referring to the streaming feature that Pondlife answered earlier


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How are redundant twin engines coupled on a helicopter?
I understand that a Eurocopter EC135 can fly on only 1 engine.
In this instance, is the dead engine power turbine still coupled to the rotor? Does it freewheel? Is there a clutch to disengage it? If not, does the power turbine draw air through the dead engine?

Welcome! Related free wheeling unit. This unit disconnects automatically the engine from the shaft. Twin-Pac is a system included in PT6T dual turbine (PT6 are not used on EC135). Using free turbines may also help (to be confirmed).

Some use a combining transmission where each engine goes into a Sprag Clutch which each feeds an output shaft connected to a gear(box) driving the rotor. When an engine fails, the sprag clutch freewheels allowing the other side to spin faster than the failed side. This is also pretty much the same in single engine helicopters which is why the rotor doesn't stop spinning when an engine fails.

There's a very simplified diagram in this accident report, see page 11 of the full report. (Doesn't show the clutches though.)

Multiple-engine configurations use multiple freewheeling units that couple the rotor with the engine and allow the first to revolve faster than the latter but not the other way around. If just one of the engines fails, the freewheeling unit for that particular engine will effectively disengage the engine from the rotor (as the rotor is now revolving faster than the failing engine), allowing the other engine(s) to drive it.
How a freewheeling unit functions on a mechanical level is described on page 6 of this document.


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What is the reason for the detour flight route from AUH to BEY?
I recently flew from Abu Dhabi to Beirut and I was intrigued by the flight route. Instead of simply avoiding Syrian airspace (as would be expected) and flying across KSA, Jordan and Israel, we flew almost to Alexandria in Egypt before hanging a big right and approaching Beirut from over the Mediterranean.

Source: FlightAware, text added
Before the Syrian war the flight used to take 3.5 hours, now it's more like 5 hours. Can someone explain to me why the flight took this route? Is it because Sinai is now considered a no-go zone? If so why not fly over Israel? Is this because it's not allowed?

@falstro no, there is no suggestion that that flight was shot down. The bomb was onboard.

@falstro I am very sorry for your dad...

@falstro: Maybe the OP could tell us what was the flight number, so that I can insert the right map. There is a FAA alert (recommendation) related to this region, only the north part, and below FL260. This alert has been relayed by EASA and other agencies. The flight shown gets round the affected area.

FYI- The flight in the image passed directly over the Sinai Peninsula, while avoiding Jordan and Israel, so overflying Sinai is not the issue.

It's for political reasons.The flight is clearly avoiding Syria and Israel. To fly a direct route between the two you must overfly either Syria or Israel.
Looking at the flight paths from the last week or so of the two airlines that service that route and it becomes very clear. The flight in question is (as it would appear from flight paths) Etihad Airways, the flag carrier for Abu Dhabi. All flights between the two from Etihad take this same circuitous route. The reason is that they can't fly over Syria or Israel. The UAE (of which Abu Dhabi is a part of) does not recognize Israel as a state. Israeli citizens are not even allowed in the UAE. Etihad does not serve Israel nor are Israelis allowed to fly on it. Many Arab states do not allow El Al to overly them so it's a safe bet Etihad is not allowed to overly Israel's territory.
In the past that would just mean flying over Syria instead. But Abu Dhabi is a member of the Arab League which has invoked sanctions against Syria. This would make it risky to fly over Syria and is most likely prohibited by the sanctions. So their only option is to go the long way around.
On the other hand, the Lebanese carrier Middle East Airlines/Air Liban also serves this route. Although Lebanon is a member of the Arab League, Lebanon refused to be party to the sanctions on Syria. Due to this they are still able to fly over Syrian airspace, so they have a much more direct route.

And it would appear that Etihad is still avoiding flying over northern Sinai since the Metrojet 9268 crash.

Thanks for the answers, both are excellent and more than answer my question. TomMcW, thanks for the info on MEA, I was curious as to why it continues to fly over Syria and now I know! It was an EY flight I was on by the way, I assume EK flights take the same route.

@Billy Yep, just looked. Emirates takes the same circuitous route.

Since most Arab countries (those who don't recognize Israel) don't allow Israeli-registered aircraft in their airspace, Israel does the same and don't allow those Arab countries-registered aircraft in its airspace.
For the same reason, every El Al flight from Israel to India/East Asia has to either fly up north (over Turkey and Kazakhstan) or down south (Gulf of Aden) before heading east.

References to NOTAMs or other authoritative documents describing the restrictions would be helpful.


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What is the reference used when measuring flight speed?
I'm a physicist who was amused by the argument in the comments of this question about gravity.  The correct answer about the affect of the accelerating reference frame of the plane on the apparent gravity depends on how the airliner's speed is measured.
I know very little about practical aviation.
When I'm on a trans-Atlantic flight moving 900 km/hr, with respect to what am I moving?  Is that my speed relative to the ground (which is also moving)? relative to the local atmosphere (which could be additionally moving due to wind)? relative to some imaginary, absolute grid hovering above the Earth's surface? relative to the "fixed stars"?
This is probably a duplicate, but I can't seem to find the right search terms...

It depends on what instrument is telling you that you are moving at 900km/hr. If you are looking at a GPS, it is ground-speed. If you are looking at the airspeed indicator, it is air speed. Remember that ground speed, is simply distance/time, even if the earth is adding or subtracting from that. Air speed is relative to the surrounding air mass and is measured via a pressure differential instrument.

Please consider adding your answer to the other question as you drive on by.  We do try to migrate these to Physics.SE, recognising that we are much better at flying aeroplanes than we are at explaining why.  Your point about the local atmosphere also moving.  When using any of the means of measuring airspeed, the movement of the atmosphere is not taken into account since for practical purposes, you, the aircraft and the atmosphere are all moving together.  The difference between your true speed through the air versus your speed over the ground is the wind.

You should be interested in the answer to Why is there a difference between GPS Speed and Indicator speed?.

One additional fun fact is that the Space Shuttle flight design teams have to agree on one or more frames of reference in order to get their calculations right. From launch until orbit, it is convenient to use a rotating frame of reference, with origin located at the launch site. Once in orbit, a geocentric, non-rotating frame is usually more handy. This conversion would add approximately 1180 km/hr velocity to a due-east launch at the equator due to the earth's rotation.

By the way, in deference to your expertise, I did of course mean your true velocity through the air versus your true velocity over the ground :)

"If you are looking at a GPS, it is ground-speed"... I understand what you mean, but strictly speaking I believe this is wrong if the flight path isn't horizontal because speed calculation takes also the vertical component of the speed into account (extreme case when the flight is vertical: GS = 0, but GPS speed is not null).

Speed of a plane is actually measured in a number of different ways, and relative to different things. Here is a summary of the different types:

Indicated Airspeed (IAS). This is the number shown on the instrument that measures airspeed, and isn't really relative to anything.
Rectified Airspeed (RAS) or Calibrated Air Speed (CAS) This is IAS corrected for known errors in the instrument and the measurement system.
True Air Speed This is the speed relative to the air around the aircraft; usually found by correcting the RAS for pressure and some other errors.
Ground Speed This is the speed relative to the ground, or a fixed point on the earth ('imaginary grid). Its the vector addition of the True Airspeed (and direction) plus the wind speed (and direction). In olden days this was estimated from the Airspeed and the wind velocity, or through dead reckoning navigation, but now it is trivially found from GPS or other navigation aids.

Planes use different ones of these for different purposes. Measurements relative to the fixed stars are not generally useful until you are in the realm of space travel.

To clarify: Indicated airspeed is essentially a measure of dynamic pressure, which is corrected to the free-field value (i.e. without the presence of the aircraft) as CAS.  CAS is equal to TAS at standard sea level air density, but decreases  as air density decreases (e.g. with altitude).  Ground speed is only the component of the aircraft's velocity along the ground plane, and does not include the vertical velocity.  Inertial speed does include all three inertial velocity components, which might be important for a helicopter or rocket!

Regarding measurements relative to fixed stars. Decades ago aviators mid-ocean at night used sextant to obtain a position & ground speed from the stars. (See Sextant used while flying and Aeronautical Sextants and Celestial Navigation)

@EricGreenwood isn't that definition for Equivalent, not Indicated airspeed? (adjusting IAS to sea level standard conditions)

There are multiple speeds referred to in aviation, some of which are not 'speed' in the traditional sense.

Indicated airspeed (IAS): This is a measure of dynamic pressure and not actual speed, measured using the pitot-static system (i.e. difference of the total and static pressure). It is a function of density and airspeed, not speed alone. Usually, the critical speeds are given in terms of IAS as the aircraft behaves similarly in same IAS.
Calibrated Airpseed (CAS): This is nothing but the Indicated airspeed corrected for instrument and position errors- still not an actual speed.
True airspeed: This gives the speed of the aircraft in relation to the sorrounding airmass. This is the speed used in the flight planning (before taking into account wind, that is) and for measuring aircraft performance. This is the speed with respect to the local atmosphere.
Ground speed: This is the speed of the aircraft with respect to the ground and as such akin to the speed of the car. This cannot be measured without an external instrument and usually, measured with GPS.

As far as air travel is concerned, speeds relative to stars are not used, as there is no need for such complexity.

I will try to add some information to the previous questions, which are correct. My intention is to provide you physical meaning, but previous answers are essentially correct.
When discussing reference frames where the speed is usually provided I will essentially summarize in 2:

Airplane reference (aka body axis): is the ideal reference frame attached to the airplane (not usually considering acceleration, but in practice should be done). The upstream speed is measure here (as the air approaches the airplane in this reference frame) and has been mentioned before as best approximation as "True Airspeed" (the other are measured and calibrated speeds).
Ground reference: is the reference located at the ground, usually located just below the airplane. That's "Ground Speed".

If you would like to know further about reference frames in the airplane, there are more reference frames (or variations of the previous one):

Body reference: just intermediate step to define other systems I am coming back to this one. Is located in the center of gravity, the x axis goes along the fuselage from cockpit to tail, z axis along gravity.
Stability reference: is the body reference rotated along y axis to make the external air speed component in the z direction 0. It is usually used to define angle of attack and compute stability of the trajectory.
Wind reference: is the stability reference rotated along z axis to make the air speed in this reference only in x direction. Formally this is where lift and drag are defined. (Notice that in body reference you are propulsed by aerodynamic force at the vertical tail plane. :)) Also, sideslip is defined as the rotation of axis system.
I have seen some literature defining lift and drag in stability reference.
I have seen in structural context to define another reference frame, x as in body reference and y axis along the wing as in body reference. This system is different to Body reference when turning, and used to compute the load factor.
It is very common to assume that the airplane is quasi-stationary, so no acceleration assumed in the frame except for highly precise calculations (or the load factor).

authors: Μάνος Μαυράκης


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How can I calculate the force on the nose gear at landing?
How can I calculate the force applied on nose gear during landing as nose gear touches the runway little after the landing gear?
I understand force calculations in this question but I think it applies on main gear. How is the force on the nose gear calculated?

It depends on the angular speed of (de)rotation, i.e. how much quickly the fuselage rotates from landing attitude to horizontal...

I think you want to start with energy then find the force.

F = ma, the hard part is figuring out how much mass is on the nose wheel. Ideally at touchdown its not more than a minor percentage of the total aircraft weight, as the goal when landing (small aircraft at least) is to keep the nose wheel up until it comes down gently on its own.

I calculated the mass on the nose wheel but i am not been able to calculate the acceleration when nose wheel touches down the runway

Calculating the loads on the nose and main landing gear isn't really different. The answer you linked is a highly simplified approach you should be able to use for the nose landing gear too, but I will provide a "less simplified" approach to calculate the vertical forces. The braking force is already greatly described there.
A simplified approach
First you need to know the impact speed and the spring and damping characteristic of your landing gear.
At touch down there are three possible cases:

Main landing gear touches the ground first. Then the airplane rotates down with an angular speed $\omega_L$. The impact speed of you nose landing gear is therefore $$V_{Impact} = \omega_L \cdot d $$ $d$: distance between the
main and the nose landing gear.
Nose landing gear touches the ground first. (For now empty, if I get time I'll write about it later)
Both landing gears touch the ground at the same time. For a glide angle $\gamma$ the impact speed will be: $$V_{Impact} = V\cdot \sin(\gamma)$$ $V$: airplane speed

The stiffness ($k$) and damping characteristic ($c$) of the landing gear depend mainly on the suspension and the tires used. Tires inflated with a wrong pressure can be really dangerous as they can increase the load on the landing gear.
Putting this data together into a mass spring damper model like in the image

gives you an equation for the nose landing gear movement: $$m\cdot \ddot{x}+c\cdot \dot{x}+k\cdot x = -m\cdot g$$
For the reduced mass $m$ in the model use the mass carried by the nose landing gear.
$$m = LM \cdot \frac{d-d_{CG}}{d}$$
$d_{CG}$: distance from the nose landing gear to the center of gravity
$LM$: aircraft landing mass
$g$: gravity constant
The initial condition for the differential equation is: $$\dot{x}(t=0) = -V_{Impact}$$
The impact occurs at $t=0$.
To calculate the force on the nose landing gear you just have tofind the maximum acceleration and add the static and dynamic loads: $$F_{max}=-m\cdot g -m\cdot \ddot{x}_{max}$$
Real world problems
While the calculations above works for really stiff wings, in reality the loads on the landing gear will be (probably) higher. This is due to the inertial forces of the oscillating aircraft structure.
If you don't understand where this forces come from try moving your arms up and down while standing: you will feel small forces on your feet. Now repeat the experiment with two 60t heavy wings as in the A380 and you'll see that the forces are not neglectable. And were do the oscillations come from? The same as with a guitar string: after a small excitation (plucking the strings/ landing impact) the structure (guitar string/ aircraft wings) begins to oscillate.
What other things doesn't this model take into account? Coupling between nose and main landing gear, rotation inertia, nonlinear tire characteristics,...

In order to calculate the force, you should start with an estimation of the angular momentum as the nose sinks and the wheel contacts the ground. That angular momentum am becomes zero as soon as the oleo leg is compressed to its maximum stroke, but that takes a measurable time, from first contact of the front wheel with the ground till the extreme compression of the oleo leg.
If you assume that the force f on the oleo leg is constant along the compression stroke, and know the time t it takes, then:
f = am/t
Of course, it's only a rough estimate, but better than nothing...
In order to calculate am, you'll need to know the mass of the plane, its moment of inertia when turning on the main wheels, and the angular velocity of that movement.